The present invention relates, in general, to apparatus and a method for the measurement of thermal conductivity of materials, and more particularly to the measurement of the thermal conductivity of polymer melts, rubber, and blends of materials at high temperature and pressures.
The measurement of thermal conductivity of materials has always been a difficult problem, with a great deal of uncertainty in the measurement. This is mainly due to factors, such as thermal contact resistance, heat losses, and heat capacity of the probe materials, which cannot be accounted for easily. Steady-state methods, because of their simple governing equations, are considered to be more accurate than the transient methods and have been adopted as the ASTM standard.
Polymer melts belong to a class of materials of intermediate thermal conductivity. Commercial polymer melts need to be measured for thermal conductivity at typical processing temperatures in the range of 150.degree. to 350.degree. C. No thermal conductivity standard has been established to date for this class of materials, and available data is inconsistent and sketchy. This is mainly because, until now, most workers have relied on the conventional "guarded hot plate" steady state device which is the ASTM standard. This apparatus is cumbersome, requires extensive sample preparation, and can take from four to twenty-four hours to reach steady state. While well worth the effort for most materials other than polymer melts, because of its accuracy, it is quite useless for polymer melts because the characteristics of these materials change, due to degradation, when exposed to high melt temperatures for extended periods of time. Also, handling molten polymers in this type of apparatus can be difficult, messy, and often hazardous. Thus, there is a need for an apparatus which will facilitate the handling of molten polymer materials and which will, at the same time, permit the measurement of thermal conductivity accurately and quickly, without the need for long-term heating to temperatures which can degrade the material being measured.
Line-source methods which measure the transient flow of heat in cylindrical geometries have been developed, and such methods, which are related in principle to hot-wire methods, have found wide application in measurements of soils, foods, and materials whose properties change with time. The line-source method has been used for measurements on polystyrene and polyethylene in the 20.degree. to 180.degree. C. temperature range. However, such apparatus has been inconvenient to use because the measurement and heater wires required for such an apparatus were embedded directly in the sample, necessitating the use of new wires for each experiment. This placed an additional degree of uncertainty to the measurement, because reproducibility could not be assured.